A sewing machine is a machine that is used to mechanically sew clothes, bags, shoes, and other sewn products, thus allowing them to be rapidly and easily made.
However, a person adjusting the sewing machine is eventually responsible for moving a sewn product, performing a sewing operation (backstitch; the same applies to the following) along a sewing line, or adjusting the sewing machine in sewing work. Thus, the efficiency of the sewing work depends on the efficiency of the sewing machine.
The basic principle of the sewing machine is as follows: under thread is provided and upper thread is pulled, so that the under thread is interwoven with the upper thread by a mechanical operation, thus making stitches on a sewn product. Particularly, an under-thread supply device for a sewing machine is the most important element in the sewing machine. The amount of the under thread provided in the sewing machine is the most important factor in determining the continuity of the sewing work, and influences the sewing efficiency. Therefore, many efforts are being made to increase the winding amount of the under thread of the sewing machine.
Nevertheless, it is believed that the width, length, and height of a current sewing machine are gradually developed to be optimal for doing the sewing work from an ergonomic point of view. Hence, those skilled in the art hesitate to change the basic structure of the sewing machine. Consequently, the development of the under-thread supply device that is a component of the sewing machine also has some limitations.
Therefore, a rotary shuttle device, which is the conventional under-thread supply device for the sewing machine, has a drawback in that the amount of the under thread provided in the device is very little, so that the under thread should be frequently replenished or replaced during sewing work. Therefore, the sewing work does not maintain continuity but is frequently interrupted, thus leading to a reduction in efficiency of the sewing work.
Further, the sewing machine consumes the upper thread of a predetermined length during a thread take-up process. In order to form the stitch on the sewn product, the upper thread should be pulled and tightened by a thread take-up lever. As a result, fine puckering (phenomenon in which the sewn product is creased or shrunk) may inevitably occur in the sewn product. Even if it is difficult to completely overcome this problem due to the properties of cloth that is the sewn product, many attempts are currently made to find a way to reduce the puckering.
A sewing process using a rotary shuttle device that is the conventional under-thread supply device for the sewing machine will be described below in brief with reference to FIG. 17.
As shown in FIG. 17, in an upper portion of the sewing machine, a needle 1 in which upper thread 12 is fitted passes through a sewn product 2 and moves up and down to a bed portion at a lower position. In the bed portion located at the lower position of the sewing machine, a hook body 3 having a hook for catching the upper thread 12 continues to rotate. A bobbin case base 4 is mounted in the hook body 3 in such a way as to rotatably accommodate a bobbin (not shown) around which the under thread 11 is wound. The bobbin case base 4 discharges the under thread 11 to the outside through an under-thread discharge hole (not shown).
As shown in FIG. 17(1) to FIG. 17(5), if the needle 1 with the upper thread 12 passes through the sewn product 2 and moves down to the bed portion as shown in FIG. 17(1), the upper thread 12 having a length of one stitch is unwound and supplied from an upper-thread spool (not shown). Thus, at the moment when the needle 1 moves up, an upper-thread loop 12′ is formed at a tip of the needle as shown in FIG. 17(2) due to a frictional force between the upper thread 12 and the sewn product 2.
If the hook body 3 rotates with the upper-thread loop 12′ being caught by the hook as shown in FIG. 17(3), the upper-thread loop 12′ surrounds and turns around the bobbin case base 4 as shown in FIG. 17(4). When the hook goes beyond a half region of a lower outer periphery of the bobbin case base 4, the upper-thread loop 12′ is removed from the hook and moves to an opposite side of the bobbin case base 4 as shown in FIG. 17(5) to be interwoven with the under thread 11 discharged from the bobbin case base 4. If the upper-thread loop 12′ is pulled to become small by the upwardly moving needle 1 and then is tightened by a thread take-up lever (not shown) to form a stitch on the sewn product 2, a toothed portion (not shown) pushes the sewn product 2 by a predetermined width. While the above-described process is repeated, the sewn product 2 is sewn (backstitched).
That is, the conventional sewing machine is configured such that the upper thread of one side inserted into a hole of the thread take-up lever pulls the upper thread wound around the upper-thread spool under a predetermined tension to cause the upper thread of a length required to form one stitch to be unwound from the upper-thread spool, by the thread take-up process, and the upper thread of the other side inserted into the hole of the thread take-up lever pulls the upper thread interwoven with the under thread under a predetermined tension, thus forming one stitch on cloth that is the sewn product.
Here, gaps are inevitably present between strands of the cloth that is the sewn product. The predetermined tension acting on the upper thread of the other side pulls the strands of the cloth that is the sewn product and eliminates the gaps between the strands, thus causing an imbalance in tension between the upper thread and the under thread and thereby leading to defective sewing, such as the puckering occurring in the sewn product. These problems should be urgently addressed.